Devices exhibiting internal polarization and apparatus for and methods of utilizing the same



3,199,086 RATUS Aug. 3, 1 H. P. KALLMANN ETAL DEVICES EXHIBITING INTERNAL POLARIZATION AND APPA FOR AND METHODS OF UTILIZING THE SAME 3 Sheets-Sheet 1 Filed NOV. 25. 1960 INVENTOR Hortmuf P Kollmonn Joseph Rennerf Fred Chernow ATTORNEY Aug. 3, 1965 H. P. KALLMANN ETAL 3,

DEVICES EXHIBITING INTERNAL POLARIZATION AND APPARATUS F'iled Nov. 25, 1960 FOR AND METHODS OF UTILIZING THE SAME 3 Sheets-Sheet 2 Movable phosphor holder and gear To variable speed mo'ror Flexible shaft Geared wheels To variable speed motor INVENTOR Harlmuf P. Kallmann Joseph Rennerf Fred Chernow BY p /w w l l l ATTORNEYS v Aug. 3, 1965 H. P. KALLMANN ETAL 3,199,086

DEVICES EXHIBITING INTERNAL POLARIZATION AND APPARATUS Filed Nov. 25, 1960 Vollage Source FOR AND METHODS OF UTILIZING THE SAME 3 Sheets-Sheet 3 To Cathode 44 Ray Oscilloscope Circuits Phosphor Sample For Image Reproducflon I 8 96 Horizontal Horizontal and 0n 2 Sweeplnpul Verfical Sweep fvertical 98 9o lnpuls 7e Sweep Generalors IN ENTOR Harlmul P Kallmann Joseph Rennerl Fred Chernow United States Patent 3,199,08fi DEVICES EXHKBITING INTERNAL POLARIZA- TION AND APPARATUS FOR AND METHODS ()F UTHLIZING THE SAME Hartrnut P. Kallmann, Bronx, N.Y., Joseph Rennert, Paranius, N.J., and Fred Chernow, Brooklyn, N.Y., assignors, by mesne assignments, to Rahn Corporation, Boston, Mass, a corporation of Massachusetts FilerlNov. 25, 1960, Ser. No. 71,627 34 Claims. (Cl. 340173) The present invent-ion relates to device utilizing the electrostatic properties of photoconductive insulating materials for image storage and reproduction, and more specifically to novel methods and apparatus for producing persistent internal polarization charge patterns of an exciting radiation or polarization pattern applied to such photoconductive insulating material, and for producing photovoltaic output signals as a means of reading out a charge pattern. By photovoltaic signal is meant any electric signal which arises upon application of radiation to the prepolarized material.

Photoconductive insulating materials sandwiched in layer form between two external electrodes, at least one of which is transparent to an exciting radiation, and polarized by the application of radiation and a direct voltage to thereby establish an internal polarization throughout the thickness of the photoconductive insulating material, have been described in US. Patent No. 2,845,348 to Kallma-nn. The electrostatic image may be subsequently detected by removal of one electrode and the use of any one of a number of techniques, the most familiar being powder development as illustrated in US. Patent Nos. 2,297,691 and 2,357,809 to Carlson.

Because theact of removing the electrodes disturbed the charge pattern and introduced distortion into the image, it was found advantageous to cover the surface of the photoconductive insulating material under the removable electrode with a plurality of small dots or islands of conductive material whereby the lifting electrode could be removed without disturbance of the charge pattern as disclosed and claimed in US. Patent No. 3,005,707 to Kallmann and Rosenberg. Several methods have been devised for the use of such material in various applications such as electro-photography, photocopying, lithography, hectography or the like. v

The present invention pertains to writing, storing, reading and erasing of information on such layers of photoconductive insulating material exhibiting persistent internal polarization in novel manners which produce output signals having improved signal-to-noise ratios, which may be photovoltaic, and which may be produced in times as short as a microsecond or less. For writing, locations on the layer may be excited with radiation having a wavelength less than about 0.6 to 0.8 micron (depending upon the specific material used) such as' radiation in the visible or ultraviolet spectral range for a short period of time to produce free charges by releasing electrons from traps. The layer is then charged with an electric field which, in accordance with one feature of the invention is an alternating field that causes a migration of the free charges to thereby establish a charge pattern corresponding to the combination of the pattern of the radiation and electric field. If the radiation is of variable intensity and the electric field homogeneous, the charge pattern will follow the variable intensity of the radiation pattern; if the radiation is homogeneous and the electric field is of variable intensity, the charge pattern will have a variable intensity in accordance with that of the applied electric field; and charge patterns of combinations of a variable radiation pattern and a variable electric field may also be produced.

Patented Aug. 3, 1965 The information so impressed in the layer may be stored at room temperatures in the absence of light for long periods of time. The stored information may be read out by conventional methods including the usual electroscopic powder development methods, or in accordance with a feature of the present invention, by measuring transient photovoltaic signals which are produced by an annihilating radiation.

Major objects of the present invention are to provide an improved device exhibiting the property of persistent internal polarization which is adapted for use as a memory or storage element in data processing or computing equipment, that has improved signal strengths whereby the signal-to-background noise ratio may be as high as 10 to 1, wherein information may be written in or read out in a fraction of a microsecond, wherein the output signal appears as a photovoltaic signal and which may be erased so that it may'again be used for the storage of further information.

Another object of the invention is to provide a method of producing mobile charge carriers. in a layer of photoconductive insulating material by irradiation and of producing a charge displacement through application of an alternating electric-field which is approximately symmetrical with respect to the central plane of the layer perpendicular to the direction of the applied external field whereby charges of the same polarity are collected under each electrode.

Another object of the invention resides in the provision of novel methods for utilizing the above mentioned charge displacement pattern for storing information by means of a latent polarization image.

Still another object of the present invention is to provide a novel means and method of Writing in and of reading out a stored electric image from a layer of photoconductive insulating (high dark resistance) material which produces a photovoltaic signal.

A still further object of the invention is to provide a system including a cathode ray tube for selectively irradiating a layer of photoconductive insulating material to store information therein and for utilizing the same cathode ray tube for releasing the information stored in the photoconductive material for detection and read out.

These and other objects of the invention will become more fully apparent from the claims, and from the description as it proceeds in connection with the drawings wherein: V

FIGURES 1, 2 and 3 show a conventional layer of photoconductive insulating material exhibiting the property of persistent internal polarization with the exciting irradiation applied in FIGURE 1, an external alternating field applied in FIGURE 2, and the resulting charge pattern after removal of the alternating voltage field illustrated in FIGURE 3;

FIGURES 4 and 5 are top plan and front side elevation views respectively of a typical box used for reading out the electrostatic image stored in the device of FIGURE 3;

FIGURE 6 is a diagram of a suitable circuit for detecting the electrostatic voltage at various locations on the charged body;

FIGURE 7 is a diagrammatic view of a system for irradiating the layer of material exhibiting persistent internal polarization with a predetermined pattern of light through a mask to write in information coded in a binary form;

FIGURE 8 is a diagrammatic view of a system for reading out information which may have been stored by the system shown in FIGURE 7, and for entering information in the material to be stored by modulation or on-ofif switching of the light beam in a cathode ray tube;

FIGURE 9 is a pictorial view of a layer of photoconductive insulating material which exhibits the property of persistent internal polarization having electrodes composed of parallel strips of conductive material on each surface, with the direction of the strips in each electrode being perpendicular to the direction of the strips in the other electrode for applying polarization to selected areas only on said layer; and

FIGURE is an exploded View of the device of FIG- URE 9 with the electrodes shown separated from the sample along with switching mean for controlling the area to which the polarization is applied.

Referring now to the drawings, FIGURE 1 shows a typical layer 16 of photoconductive insulating material sandwiched between two layers 12 and 14 of conductive glass which serve as electrodes. Such photoconductive insulating materials may be inorganic phosphors which display photoconductivity, such as zinc sulphide, cadmium sulphide, zinc and cadmium ulphide activated with suitable material such as silver, copper, or gold, as well as alkali halides, anthracene, chrysene or the like, an essential condition being that these photoconductors exhibit high resistance when not illuminated or subjected to radiation. All photoconductive insulating materials are believed to exhibit persistent internal polarization after irradiation and polarization with an electric field.

The powdered materials conventionally used may be mixed with a binder such for example as cellulose nitrate, silicones or epoxy resin-s. Such powdered material may be prepared on layers of glass that have been tin oxide coated to make them conductive to serve as the electrodes. Preparation may consist of the steps of depositing the photoconductlve insulating material in a suitable matrix on the electrode so that the sandwich of the electrodephotoconductive insulating material-electrode is formed. The edges are then sealed with an epoxy resin, leaving portions of the electrodes bare for making electrical contact. The following polymer have been used and found satisfactory as a matrix: a proxylin-base cement (Ducohousehold cement), epoxy resin (Shell Epon 828) and silicone resin (G.E. S.R82).

For use as a matrix the Duco is dissolved, 2%6% by volume, in amyl acetate, and the photoconductive insulating material powder may be then added to the solution. After mixing vigorously for two to four minutes, the solution is poured into a Petri dish which contains an electrically conductive transparent glass, conducting side face up. The powder is allowed to settle out, and then the supernatant solution of Duco in amyl acetate is drawn off. When the powdered layer is dry the second glass electrode is set in place. For the silicone resin matrix, the same procedure may be used as in preparing the Duco matrix. A solution of 2l0% G.E. S.R.82 in xylene is used. After this sample is dry, it is baked in an oven for one hour at 150 C. Then the seconod electrode is applied to the phosphor layer.

The epon resin samples may be prepared by mixing the resin with 30% curing agent and powder for fifteen minutes. The resulting slurry is then deposited on a glass electrode. A second glass electrode is immediately applied, and the two electrodes are squeezed together to produce a unifoorrn layer between them. The sandwich formed is then left to harden.

Such photoconductive insulating materials are sensitive to radiation having wavelengths less than about 0.6 to 0.8 micron such as visible light and also ionizing corpuscular raditions such as alpha and beta rays and electr-amagnetic radiation which is absorbed in the material such as gamma rays, X-rays, ultra-violet. Such radiation causes free charges to be produced by freeing electrons so that upon subsequent application of an electric field, the free charges may be removed from the layer of photoconductive insulating material. Such materials are also sensitive, though in a different way, to infrared radiation with wave lengths up to 2.5 microns. Such longer wavelength radiation and heat has the effect of causing any 4. free charges to be lost apparently by causing the freed electrons to be subjected again to the recombination with positive charges.

Electrodes 12 or 14 may be transparent to the radiation to be detected, but not necessarily transparent to visible light. Also, it is possible that only one of the electrodes is transparent to the radiation to be detected in certain applications, though for many practical applications, both electrodes may be of substantially identical construction.

In the prior work of persistent internal polarization, it has been the practice to apply an electric field concomitantly with or subsequent to the irradiation of the photoconductive insulating material. The electric field applied was a direct voltage in all such prior work to the best of our knowledge. The charge pattern thus extended throughout the thickness of the layer between the electrodes. After the polarization pattern was established, one of the electrodes was removed to thereby expose a surface of the charged material and make available the electrostatic charge so that it could be detected.

Referring now to FIGURES 4 and 5, a typical box is illustrated whereby the sample layer 10 is supported by electrode 12 (see FIGURE 5) by a suitable bracket 2i) and irradiated through a window composed of filter 22, shutter 24, and glass 26. The second electrode 14 is shown in FIGURE 5 in its lifted position as it would be after the sample layer 10 had been polarized and when it was desired to read out the stored signal.

As shown in FIGURE 4, the sample holder brackets 20, which may be in the form of a rectangle, are supported by shaft 30 which contains a helical thread and is adapted for rotation to thereby move the sample in a fore and aft direction across the box. The scanning probe 32 is mounted on shaft 34 which is also provided with a helical thread to thereby move probe 32 across sample 10 in the lateral direction. By this arrangement, probe 32 may be moved across the exposed surface of sample 10 to detect the electrostatic charge at various positions on the sample, and the sample may be moved at the end of the path of travel of probe 32 whereby the charge pattern of the entire surface may be detected.

In FIGURE 6, a circuit for reading out the polarization charge is illustrated. Sample 10 is shown with electrode 50 grounded and the scanning probe 32 connected to a three position switch 40 which connects movable probe 32 to the input terminal of vacuum tube 42. The output signal is then available across cathode resistor 44 on lead 46 and may be applied to suitable indicating device such as a cathode ray oscilloscope. By the foregoing arrangement of parts, it is possible to detect the magnitude of the electrostatic image stored at various locations within the sample 10 to thereby determine the relative intensity of the stored charge in the various portions.

Alternatively, an electroscopic powder may be used to develope an image of the charge pattern in lieu of the scanning probe 32 if desired. Also, where the conductive dots or islands are provided on the upper surface of layer 10 as viewed in FIGURE 5 under the lifting electrode, improved image resolution is attained through application of a further flash of radiation after the electrode is lifted, which transforms the internal polarization charge to a free charge on the island electrodes as explained in the above-identified Patent No. 3,005,707. Such free charges may then be detected by any of the conventional methods or by the novel method discussed below wherein a photovoltaic signal is produced.

In accordance with one of the principal features of the present invention, the external electric field applied to the layer of photoconductive insulating material is an alternating field rather than direct field as previously used. With reference to FIGURES 1, 2 and 3, the initial radiation is applied through electrode 12 while the electrodes are grounded to thereby produce mobile charge carriers in the body of layer 10. We have found that these mobile charge carriers are electrons that are trapped in the photoconductive insulating material, and that radiation from an ordinary tungsten lamp as for a time of approximately thirty seconds, or from any source of radiation having a wave length shorter than that of visible light, is effective for producing mobile charge carriers. In the case of zinc sulphide, ultra-violet light is most eifective; with zinc-cadmium sulphide, blue light is preferred; and with cadmium sulphide, green light is effective.

Thereafter, even as much as a day later, an alternating voltage may be applied to the sample as is shown in FIGURE 2, to thus cause a separation of the charge carriers. An electrical field having a frequency of 60 cycles per second with a voltage of approximately 250 volts may be applied for a period of time on the order of fifteen seconds. The eflect of the alternating electric field is to cause the mobile charges or electrons freed in the photoconductive insulating material by the exciting radiation to be pulled away from the layer to thereby provide a positive charge near the surface of layer 10 under each electrode, which persists after removal of the alternating field as is illustrated in FIG- URE 3. Therefore, any type of alternating voltage, which may for example be merely a repeated reversal of a direct potential is suflicient to cause the separation of the charge carriers. Frequencies up to kilocycles may be used, though the upper limit is probably determined by the mobility of the charge carriers in the particular photoconductive insulating material used. Where both the radiation and the electric field are homogeneous, the charge displacement is approximately symmetrical with respect to the central plane indicated by line 15 in FIGURE 3. The strength of the charge pattern may be more concentrated than if a uni-directional polarizing voltage. is used and therefore a better signal-to-noise is obtained.

The alternating fields have a polarizing effect only when the sample is pre-excited by an energizing radiation. It is possible of course to apply the alternating field during irradiation, but the radiation must be turned off prior to removal of the electric field.

The charge displacement or separation of charge carriers resulting from application of the electric field persists in the dark for hours, and may be detected by any of the conventional methods.

To annihilate the charge separation, the electrodes are connected in a circuit to ground and a layer is exposed to a flash of radiation shorter than about 0.8 micron.

The magnitude of the polarization may be detected by having a current sensitive element such as resistor 52 in the circuit for electrode 50 (see FIGURE 6) so that when the electrons are replaced back into the layer of photoconductive insulating material the current required may be detected. The magnitude of this current gives an indication of the magnitude of the charge which had previously been stored in the device. The radiation to effect the annihilation of the charge separation in one example resulted from a light flash of 0.7 microsecond di rected onto the transparent electrode. Accompanying this flash of radiation, a voltage signal in the order of 2 volts was obtained across resistor 52 on lead 58. A second similar light flash exposure released only a voltage signal of about 0.35 volt. This demonstrates that the first light flash released most of the polarization.

After the foregoing exposures for annihilating the charge separation, the sample was irradiated through the other electrode with a similar flash of light. This flash released a voltage signal of 1.5 volts which is only slightly smaller than the voltage release obtained with the first flash on the first side. The light used was a strongly absorbed radiation such as violet or ultra-violet radiation. Such radiation is strongly absorbed by the photoconv of the pattern of second radiation.

ductive insulating material so that it essentially releases polarization only on the surface where. the radiation impinges. Thus, the polarization on each side of the layer may be independently detected. Where the radiation is a weakly absorbed type, the elfect of the radiation is not localized merely on the side which the radiation impinges, but will erase the polarization on both sides of the layer 10.

With the principles described above, several distinct methods of utilizing the device in accordance with the present invention have been devised.

One method involves the application of an exciting radiation having an image pattern to be depicted followed by the application of an external alternating voltage. The initial radiation produces mobile charge carriers, the concentration of which is a function of the intensity of the radiation, while the application of the homogeneous alternating voltage field causes a seperation of charge carriers. The magnitude of the electric field is sufficient so that mobile charge carriers or free electrons are displaced from the layer of photoconductive material. In this case, the radiation of highest intensity produces the largest charge, and if one electrode is removed as illustrated in FIGURES 4 and 5, then the charge on the surface may be developed by use of the conventional electrophotograhpy techniques or the charge may be measured by a scanning probe and charge indicator such as an electrometer.

In accordance with another feature of the invention, it is possible to significantly increase the magnitude of the signal detected by the circuit in FIGURE 6 if layer 10 is irradiated through electrode 50 by a releasing radiation whereby the charge separation is annihilated by a current flowing through resistor 52, switch arm 54, and lead 56 to thereby provide an output photovoltaic signal on lead 58. To detect different charge separations in different portions of layer 10, each portion may be irradiated separately with a beam of light of substantially uniform intensity.

A second distinct method of producing a charge displacement within the layer 10 of photoconductive insulating material consists of first applying a homogeneous exciting radiation to produce the mobile charge carriers and then applying a homogeneous electric alternating field to thereby produce a charge displacement that is approximately symmetrical with respect to the central plane of the layer perpendicular to the direction of the applied external field as indicated in FIGURE 3. This charge displacement may be retained until it is desired to detect a radiation pattern having an image which is to be depicted. A second radiation is then applied to cause partial annihilation of the separation of the charge carrier at a time when there is no electricfield applied to layer 10 and when the electrodes are connected to a circuit which may be connected to ground. After this step of irradiation, the layer may again be stored for considerable length of time before reading out the polarization pattern. The remaining polarization pattern in this case will be negative of the image This means that if the second radiation pattern had a portion which was dark, the full charge separation would be retained in that portion of layer 10 and if full intensity radiation is applied to a portion of layer 10, the charge separation at that portion of the layer would be substantially annihilated. Because a light flash having incident energies in the order of ergs/cm. and of duration less than one microsecond is suificient to annihilate the charge separation, extremely high sensitivity is achieved.

When it is desired to read out the remaining charge separation with an arrangement such as shown in FIGURE 6, switch contact 54 would be transferred so that the circuit connected to the electrode would also contain resistor 52 to thereby generate the photovoltaic signal on lead 58 when the light flash is applied to layer 10 through electrode 50 to thereby annihilate any remaining charge. To detect the magnitude of the stored charge in various portions of layer Ill, the electrode may be illuminated with a spot of light covering only the portions of the electrode wherein the image charge is desired to be detected. The image resolution is thus determined largely by the size of the spot of light which is used to annihilate the remaining charge.

One of the advantages in producing a photovoltaic output resides in the fact that both electrodes may be permanently secured to layer as shown in FIGURE 5. This makes fabrication of the sample much easier and eliminates the need in large part for the conductive island or dot electrode under the lifted electrode as disclosed in the above mentioned Patent No. 3,005,707. However, it is still possible to use such constructions with the present invention should it be so desired.

The present invention is particularly well adapted for use as a memory device. Apparatus as diagrammatically illustrated in FIGURE 7 may be used for entering information of a binary notation into the photoconductive insulating material in accordance with either of the above two methods. In the first methods, the sample 1d of FIGURE 1 may be mounted immediately adjacent mask 60 which is provided with a horizontal row 61 of positions that may be either transparent or opaque to the radiation emitted by trace 62 on the face of cathode ray tube 64. As trace 62, which may comprise a vertical line, moves across the face of cathode ray tube 64, the various portions of the mask are irradiated. Those portions of the layer 10 exposed to the radiation through a transparent portion of mask 60 will thus have free charges produced which may be displaced under the action of a subsequently applied external alternating voltage field. The charge separation resulting from application of the electric field then persists until such time as it is desired to read out the charge displacement.

The charge displacement may then be read out with mask ea removed by the same radiation emanating from trace 62 as it moves across the face of cathode ray tube 64 when the electrode facing mask 60 is connected to a circuit through resistor such as illustrated in FIGURE 6 to provide a photovoltaic output on lead 58.

After the stored signal is read out, the sample may be completely de-excited by infrared light or heat. Infrared radiation from about 0.6 to 2.5 microns or heating as in an oven up to a temperature of 100 C. or higher quenches any free charges remaining in the photoconductive insulating material. After quenching, layer it is then ready for re-excitation by again placing mask 60 between the sample to be irradiated and trace 62 on cathode ray tube 64.

The second method as described above is simpler to use in this system. The photoconductive insulating material is uniformly irradiated with a homogeneous pattern of radiation and a homogeneous electric field is also applied to thereby provide the charge displacement which is approximately symmetrical with respect to the central plane of the layer perpendicular to the direction of the applied external field. Then the sample is irradiated with light from trace 62 of cathode ray tube 64 through the charge displacement which is approximately symmetrical with respect to the central plane of the layer perpendicular to the direction of the applied electric field. Then the desired signal may be entered into the unit by using the same mask 6% or another mask with a different pattern of areas which are selectively transparent and opaque to the radiation from beam 62.

Referring now to FIGURE 8, there is a further system illustrated for utilizing either of the two methods described above for impressing information in layer 10 where it may be stored until it is desired to read it out.

In this system cathode ray tube 76) is used to produce a spot '72 which is caused to traverse a field 74 by conventional horizontal and vertical sweep generator 76 to thereby scan the surface of layer 10, the light from the spot passing through lens 73. The electrodes of layer 10 are connected through leads 8% to amplifier 82. The output signal from amplifier 82 on lead 84 may be connected to a display scope 85 as the Z-input to thereby modulate the intensity of the beam on that scope. The sweep for field 8% on display scope 8:) may be controlled by signals on lead from horizontal and vertical sweep generator 76 to thereby be synchronized with the sweep on cathode ray tube 70. A monitor scope 92 may be provided to also receive signals from amplifier 82 on lead 94. Its horizontal sweep may be synchronized with the horizontal sweep on scope 7t} and the input signal on lead 94 connected to the Y-axis to thereby provide a signal such as illustrated in FIGURE 8 which corresponds to areas of charge separation and no charge separation in layer it).

In this embodiment, the variable radiation pattern may be provided by modulating the intensity of the beam producing the trace spot 72 in cathode ray tube 763. Where the sample is to be used as a memory unit for storing binary information, modulation of the beam in cathode ray tube 70 may be simply an on-otr' type mask 69 in spe cial localized areas determined by the location of the transparent and opaque portions of strip 61. Only in those areas which are irradiated by the light from trace 62, is the change separation annihiliated, and the polarization remains in the un-irradiated areas. In this way, information is impressed on layer 10 of photoconductive insulating material.

The reading out of information stored in the sample is accomplished with mask 60 removed. Thus, with the electrode connected through a circuit including a current responsive device such as resistor 52, a photovoltaic signal proportional to the current necessary to annihilate the charge separation remaining in layer it) will be obtained on lead 58. At a location on layer it) under a portion of mask 60 which has an opening transparent to the radiation from beam 62 during the second exposure for impressing information in the layer, the charge separation will have already been annihilated and therefore when this location on layer 1% is again illuminated during the read out step, there will be a very small or nearly zero photovoltaic signal produced. On the portion of layer It? which had been shielded from light beam 62 by an opaque portion of mask 66 during the second step of irradiation, there will have been no annihilation of the charge separation and therefore during the readout step when the light from beam 62 is again applied to layer It a maximum photovoltaic signal on lead 58 is generated when the light beam is on a previously shielded portion of layer 16. Thus, a binary type signal may be generated from the storaeg unit.

One further advantage of this last method is that the read out process has the effect of annihilating the entire charge separation on layer It to thereby also serve as the exciting radiation. Thus, to re-use layer iltl, it is necessary only to momentarily apply an alterating electric field to again produce pulsating arrangement whereby the spot 72 is caused to selectively illuminate or irradiate predetermined portions of layer ill to thereby supply mobile charge carriers only at such irradiated locations. Transfer of switch contacts d8 applies a homogeneous electric field from source 96 to cause a charge separation to be generated which corresponds in location to regions which were illuminated by spot 72.

Alternatively, layer It? may be irradiated with a beam '72 of uniform intensity to thereby provide a homogeneous excitation of the layer. The homogeneous electric field may also be applied as before whereby the charge displacement is approximately symmetrical with respect to the central plane of the layer perpendicular to the direction of the applied external field. Then, modulation of the beam resulting in spot 72 being switched on and oil, may be used to selectively annihiliate the charge separation to thereby store in layer 10 the coded information.

In either case, the coded information stored in layer 10 is read out by using a steady beam providing a spot 72 9. of susbtantially constant intensity to thereby provide the photovoltaic output signal on leads 80 to amplifier 82. Where the second method of partial or substractive anni' hilation is used, after each final read out, switch 98 is transferred to thereby momentarily apply the electric field to again provide a charge displacement which is approximately symmetrical with respect to the central plane of the layer perpendicular to the direction of the applied external field, since the final read out irradiation step is effective to serve also as the exciting radiation.

Information may also be stored in layer in accordance with the present invention by applying an electric field having a pattern of variable intensity rather than being homogeneous as was assumed in all of the foregoing examples. The variable intensity electric field may be used with either homogeneous radiation or combined with radiation having an image pattern.

For purposes of utilizing the layer 10 as a memory element, binary information may be easily stored in layer 10 by utilizing an electrode structure such as that shown in FIGURES 9 and 10 instead of the homogeneous electrodes as previously described in connection with all of the other figures. In FIGURE 10, the layer 100 of photoconductive insulating material is sandwiched betwene two electrodes 102 and 104. These electrodes may be formed from conductive glass wtih small non-conductive strips dividing the electrode into strips 106 of conductive material. Strips 106 on one electrode are all parallel to each other and preferably disposed perpendicularly to strips 108 on electrode 104. Each of strips 106 is connected to different contacts on switch 112 and each of strips 104 is connected to separate contacts on switch 114. The moving arms on switches 112 and 114 are connected to opposite terminals of a source 116 of alternating voltage. Layer 100 of the photoconductive insulating material is sandwiched between electrodes 102 and 104 and sealed in position as with an epoxy resin. The assembled element then has the appearance as illustrated in FIGURE 9.

With the sample as illustrated in FIGURES 9 and 10, the exciting radiation may be applied in a homogeneous manner as by the light from spot 72 on cathode ray tube 70 (FIGURE 9) to thereby provide mobile charge carriers and the electric field than applied from the source of alternating current to each of the horizontal conducting strips 106 on electrode 102 in a sequential arrangement. While switch arm 112 remains on one of the contacts connected to one of the strips 106 on electrode 102, switch 114 may be moved to each of its positions and switch 118 optionally closed in accordance with the information to be stored in layer 110 to thereby selectively polarize the various portions of layer 10. The electrical field therefore is effective to produce a charge displacement that is approximately symmetrical with respect to the central plane of the layer perpendicular to the direction of the applied external field only at those areas of cell 110 where the electric field is applied. In the remaining portions of layer 100 which are not polarized, there is no separation of the charge carriers.

For read out, the system shown in FIGURE 8 may be used with the light from spot 72 of cathode ray tube 70 having a uniform intensity to thereby sequentially scan the various areas of the memory unit and produce a photovoltaic output voltage at those regions where the electric field had been applied. A considerably smaller or no photovoltaic signal appears from the memory unit when the portions which are not polarized are illuminated by the light from spot 72.

The step of reading out the radiation not only causes the annihilation of the charge separation pattern previously stored in memory unit 10, but also may be used to re-excite the memory unit whereby new information may be stored in the unit by selective application of the electric field as by means of the switching circuit shown in connection with FIGURE 10.

It is obvious that any of the other methods of reading out a stored electrostatic image may be used where the polarization is effected by application. of a selective electric field as described above in connection with the processes where polarization is effected by application of a selective radiation pattern. i

It is also possible to use the novel memory device described above to provide an output photovoltaic signal at positions which have two input signals, and no photovoltaic voltage output at locations where only one input or no input signal i applied. In such an embodiment, the initial exciting radiation may be supplied through a mask such as mask 60 of FIGURE 7 or by on-olf switching of the beam in cathode ray tube 70 as in FIGURE 8, or by a combination thereof to thereby provide selective excitation of layer 10. Then with the electrode pattern configuration as shown in FIGURES 9 and 10, the electric field may be applied in a further selected pattern whereby a charge separation is produced in the sample layer 10 only at the positions which receive both the exciting radiation and the electric field. This information may then be read out and detected or, a further input may be used frared radiation.

to reduce the number of areas which contain polarization by again irradiating the layer 10 through a further mask 60 as in FIGURE 7, or by modulation of the electron beam in the cathode ray tube as in FIGURE 8, to thereby selectively annihilate the charge pattern in areas where the charge separation had been previously produced. Then, with the further step of irradiating layer 10 with a light beam of uniform intensity, the photovoltaic output signal to amplifier 82 will occur only where a charge separation remains. Thus, the memory unit of the present invention provides a great deal of flexibility which is not present in many of the conventional memory units.

The speed of writing in and reading out information is limited only by the time required to produce the necessary light flashes and the switching for providing the desired electric field. There are no moving parts since the switching may be accomplished electronically and the write in or information by variable irradiation patterns may be accomplished with the unit remote from the equipment required to produce the electric field. Thus, the layer 10 may be exposed as in photography to an image and analog signals produced as well as using layer 10 for storing only information of the binary type.

The present inventionis also useful for detecting in- Infrared radiation has the effect of deexciting the photoconductive insulating material by causing charge carriers which had been excited, to recombine and thereby to become incapable of being separated from the material by the application of the polarizing electric field. Thus, exposure of an excited layer of material to the visible or ultra-violet radiation, or other exciting radiation having a wave length less than about 0.6 to 0.8 micron results in the creation of free charges near the electrode and if infrared radiation having wave lengths up to 2.5 microns or heat is selectively applied to the layer, the free charges previously created are trapped. Thereafter, when an alternating voltage field is applied, only those areas which were sheltered from the infra-red radiation or heat have charge carriers whichcan be separated. Therefore, when the signal is detected by conventional techniques or read out by application of further radiation to annihilate the charge, the charge separation is present only in those areas which had not been subjected to the infrared radiation or heat.

The present invention thus makes possible the detection of infrared radiation in a very convenient manner by providing a pre-excited layer of photoconductive insulating material and then leaving it in an area where it is desired to detect the presenceof any infrared radiation without any connections to any electrical circuit. Then at a latter time, the layer can be removed from its detecting position and polarized with the electrical field and the charge separation observed to thereby indicate whether there had been exposure to infrared energy.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

What is claimed and desired to be secured by United States Letters Patent is:

1. In a method of producing an electrostatic charge pattern in a layer of photoconductive insulating material exhibiting persistent internal polarization after exposure to radiation and a polarizing voltage, there being electrodes on opposite sides of said layer with at least one of said electrodes being transparent to the irradiation applied to said layer, the steps of producing excited charges in said photoconductive insulating material by applying an exciting radiation to the layer; then applying an alternating voltage to the electrodes on opposite sides of said layer to produce a charge separation caused by charge migration whereby charges of the same polarity are collected at the surfaces on opposite sides of said layer under each electrode; and removing the alternating voltage to thereby provide an electrostatic charge pattern corresponding to the charge separation.

2. The method as defined in claim 1 wherein the excited charges are electrons and the charge migration results from movement of said electrons to thereby provide a positive charge at the surfaces on opposite sides of said layer under each electrode.

3. The method as defined in claim 1 wherein the exciting radiation has an intensity pattern corresponding to an image to be depicted, and the electric field produced by the alternating voltage is homogeneous.

4. The method as defined in claim 1 wherein the exciting radiation and alternating voltage field are homogeneous to thereby provide a charge displacement that is approximately symmetrical with respect to the central plane of the layer perpendicular to the direction of the applied voltage, and further comprising the step of applying a releasing irradiation after removal of the alteranting voltage from said electrodes, having a pattern corresponding to the image to be depicted to thereby annihilate the charge displacement in part to establish a charge distribution pattern corresponding to the negative image of the releasing irradiation.

5. The method as defined in claim 1 wherein the exciting radiation is homogeneous and the alternating voltage is applied only to selected areas of said layer to thereby produce charge separations in a pattern corresponding to the application of the electric field.

6. The method as defined in claim 1 wherein the exciting radiation is homogeneous and thereafter applying infrared radiation before applying the alternating voltage to thus de-excite and reduce the number of n charges in said layer. 7. The method as defined in claim 6 wherein the infrared radiation has an image to be depicted and the final charge pattern in the layer corresponds to the negative image of the infrared radiation pattern.

8. The method as defined in claim I wherein the image is reproduced as a photovoltaic signal by the further step of applying a releasing radiation of substantially uniform intensity through said radiation transparent electrode to annihilate the charge separation and detecting the current through a circuit to said electrode as a measure of the charge separation.

9. The method as defined in claim 8 wherein the releasing radiation is directed to cover only a portion of said layer and is controlled to scan over said layer in an orderly fashion to thereby provide a current through said circuit which is in time correspondence with the magni- 12 tude of the charge in the portion of the layer then subjected to said releasing radiation.

10. In a method of producing a photovoltaic signal from a layer of photoconductive insulating material exhibiting persistent internal polarization after exposure to raditation and a polarizing voltage, there being electrodes on opposite sides of said layer with at least one of said electrodes being transparent to the irradiation applied to said layer, the steps of: producing free charges in said photoconductive insulating material by applying an exciting radiation to the layer; then applying an alternating voltage to the electrodes on opposite sides of said layer to produce a charge separation caused by charge migration whereby charges of the same polarity are collected at the surfaces on opposite sides of said layer under each electrode; removing the alternating voltage; connecting the radiation transparent electrode in a circuit having current responsive means; and irradiating the layer to thereby annihilate the charge displacement and produce a current having a magnitude indicative of the charge migration.

11. The method as defined in claim 16 wherein the exciting radiation has a pattern of variable intensity over the surface of said layer and the annihilation radiation is of substantially uniform intensity.

12. The method as defined in claim 11 wherein the exciting radiation is applied to said layer through a mask having sections which are selectively transparent and opaque to said radiation.

13. The method as defined in claim 11 wherein the exciting radiation comprises a spot of radiation much smaller than the area of said layer and the intensity of the radiation spot is altered as the spot is moved to different positions on the layer.

14. In a method of generating a photovoltaic signal from a layer of photoconductive insulating material exhibiting persistent internal polarization after exposure to radiation and a polarizing voltage, there being electrodes on opposite sides of said layer with at least one of said electrodes being transparent to the radiation applied to said layer, the steps of: applying a substantially homogeneous exciting radiation to the layer to produce free charges in said photoconductive insulating material; then applying an alternating voltage to the electrodes on opposite sides of said layer to produce a charge separation caused by charge migration whereby charges of one polarity are collected at the surfaces on opposite sides of said layer near both electrodes and the charge displacement is approximately symmetrical with respect to the central plane of the layer perpendicular to the direction of the applied alternating voltage; removing the alternating voltage; then connecting the radiation transparent electrode in a circuit and applying a radiation pattern through said radiation transparent electrode to cause selective annihilation of the charged displacement; and thereafter finally irradiating the layer through the radiation transparent electrode with a source of substantially constant intensity to thereby further annihilate the charge displacement in said one side of said layer and produce a current in said circuit having a magnitude indicative of the charge displacement annihilated by said final irradiation.

15. The method as defined in claim 14 wherein the radiation causing selective annihilation of the charge displacement is applied to said layer through a mask having sections which are selectively transparent and opaque to said radiation.

16. The method as defined in claim 14 wherein the radiation causing the selective annihilation of the charge displacement comprises a spot of radiation much smaller than the area of said layer and the intensity of the radiation spot is altered at different positions on the layer.

17. The method as defined in claim 14 wherein the second step of irradiating the layer utilizes a strongly absorbed radiation whereby only the charge separation under one electrode is annihilated.

18. In a method of producing aphotovoltaic signal corresponding to the intensity of an electrostatic charge in a layer of photoconductive insulating material exhibiting persistent internal polarization after exposure to radiation and a polarizing voltage, there being electrodes transparent to the irradiation applied to said layer on opposite sides of said layer, the steps of: producing free charges in said photoconductive insulating material by applying an exciting radiation to the layer; then applying an alternating voltage to the electrodes on opposite sides of said layer to produce a charge separation caused by charge migration whereby charges of one polarity are collected at the surfaces on opposite sides of said layer under both electrodes; removing the alternating voltage; connecting the electrodes in a circuit and exposing opposite sides of said layer to radiation strongly absorbed by said insulating material and of varying intensity to cause independent selective annihilation of the charge displacement under each electrode in accordance with the pattern of the radiation received; and irradiating the layer with radiation of substantially uniform intensity to thereby annihilate the charge displacement on each side of the layer independent of the charge displacement on the other side of the layer and thereby produce separate currents having magnitudes indicative of the charge migrations under each electrode.

19. The method as defined in claim 18 wherein the last-mentioned irradiation is strongly absorbed to thereby be effective on only one side of the layer, and the layer is irradiated on both sides in a sequential manner to produce said separate currents.

20. In a method of producing a photovoltaic signal corresponding to variations in intensity of an applied infrared radiation on a layer of photoconductive insulating material exhibiting persistent internal polarization after exposure to radiation and a polarizing voltage, there being electrodes on opposite sides of said layer with at least one of said electrodes being transparent ot the radiation to be detected, the steps of: producing free charges in said photoconductive insulating material by applying an exciting homogeneous radiation to the layer; then applying infrared radiation to said layer to quench said free charges; thereafter applying an alternating voltage to electrodes on opposite sides of said layer to produce a charge separation caused by charge migration in areas not quenched by infrared radiation whereby charges of one polarity are collected at the surfaces on opposite sides of the layer under both electrodes; removing the alternating voltage; connecting the radiation transparent electrode in a circuit having current responsive means; and irradiating the layer to thereby annihilate the charge displacement under the electrode and produce a current having a magnitude indicative of the charge separation annihilated.

21. The method defined in claim 20 wherein the final irradiation is directed to only a portion of one side of the layer at any one moment and further including the step of scanning said layer in a predetermined raster to thereby produce current pulses of varying magnitude according to the charge displacement annihilated by the final irradiation at the portion of the layer irradiated at said one moment.

22. A memory unit comprising: a layer of a photoconductive insulating material exhibiting the property of persistent internal polarization after exposure to radiation and a polarizing voltage; means for writing information into said layer by producing mobile charges through irradiation of the layer and separation of the mobile charges by an external electrical field, the variation of magnitude of the separation of the charges at diiferent positions on said layer serving as the information; and means for producing a photovoltaic signal proportional to the separation of the charges at said difierent positions on said layer comprising means for individually illuminating said different positions and means responsive to l d the voltage signals produced for detecting said information.

23. The memory unit as defined in claim 22 wherein the variation in magnitude of the charge separation is provided by the radiation applied to said layer having variable intensity and the electric field applied to the layer is homogeneous.

24. The memory unit as defined in claim 23 wherein the variable intensity radiation is eifected by means comprising a source of substantially constant intensity radiation and a mask positioned between said source and said layer, said mask having portions which are transparent and opaque to the radiation applied through said mask.

25. The memory unit as defined in claim 23 wherein the variable intensity radiation is effected by means comprising a source of radiation having a beam output, and means for modulating the intensity of the beam.

26. The memory unit as defined in claim 22 wherein the variations in the magnitude of the charge separation is provided by the electric field applied to said layer having a variation in magnitude and the radiation applied to said layer is homogeneous.

27. The memory unit as defined in claim 26 having electrode structures on opposite sides of said layer composed of a plurality of strips of conductive material; the strips on each electrode being parallel to one another and lying in a direction perpendicular to the direction of the strips on the other electrode, and the electrode on one side of said body being substantially transparent to radiation to which the layer is photo responsive, and means for selectively applying a voltage source to produce said electric field including switching means for connecting said voltage source to a strip of conductive material on each electrode simultaneously.

28. In combination, a body of a photoconductive insulating material exhibiting the property of persistent internal polarization after exposure to radiation and a polarizing voltage; an electrode structure on each side of said body comprising a layer of insulating material and a plurality of strips of electrically conducting material, the conducting material being on each side of said body between said layer and a surface of said body, the strips on each electrode being parallel to one another and lying in a direction perpendicular to the direction of the strips on the other electrode, the electrode on one side of said body being substantially transparent to radiation to which the photoconductive insulating material is photo responsive; means for populating electron traps in the photoconductive insulating body comprising a source of radiation positioned to irradiate the body through said radiation transparent electrode; and means to polarize selected portions of said body by pulling electrons outwardly from both sides of said body comprising a source of alternating voltage having two terminals and circuit means for connecting a selected strip on one electrode to one terminal and a selected strip on the other electrode to another terminal of said alternating voltage source.

29. In combination, a layer of a photoconductive insulating material exhibiting the property of persistent internal polarization after exposure to radiation and a polarizing voltage; an electrode structure on each side of said body comprising a layer of insulating material and a plurality of strips of electrically conducting material, the conducting material being on each side of said body between said layer and a surface of said body, the strips on each electrode being parallel to and spaced from one another and lying in a direction perpendicular to the direction of the strips on the other electrode, the electrode on one side of said body being substantially transparent to radiation to which the photoconductive insulating material is photo responsive; means for populating electron traps in the photoconductive insulating body comprising a source of radiation positioned to irradiate the body through said radiation transparent 15 electrode; means for pulling electrons outwardly from both sides of said body comprising a source of alternating voltage having two terminals, circuit means for connecting a selected strip on one electrode to one terminal and a selected strip on the other electrode to another terminal of said alternating voltage source and switching means for disconnecting said electrodes from said alternating voltage source to thereby polarize a selected location on said body determined by the overlapping area of the selected electrode strips; and means for again irradiating said body through said radiation transparent electrode to read out the magnitude of the stored charge.

30. The combination as defined in claim 29 wherein said read out means comprises a circuit means for connecting all of the strips of one electrode to a separate electrical conductor lead, means for connecting said electrical conductor lead to a current responsive indicator means, and the irradiation means for reading out the stored charge individually irradiates each of the plurality of locations on said body.

31. A method of reusing a layer of photoconductive insulating material exhibiting the property of persistent internal polarization after exposure to radiation and a polarizing voltage as a memory unit producing a photovoltaic output, comprising the steps of: producing a charge separation in said layer resulting from migration of mobile charges with the charge displacement being approximately symmetrical with respect to the central plane of the layer between electrodes provided for applying an electrical field across the layer whereby charges of the same polarity collect at the surfaces on opposite sides of the layer under each electrode; irradiating said layer through one of said electrodes with radiation of variable intensity to selectively annihilate said charge separation to thereby store information in said memory unit; then reading out the stored information by irradiating the layer through the same electrode with radiation of substantially uniform intensity and concomitantly detecting and measuring the current required to annihilate the remaining charge separation as the output signal from said memory unit; and thereafter applying an electric field across said layer to again produce a charge separation whereby said layer may again be used as a memory unit.

32. A method of reusing a layer of photoconductive insulating material exhibiting the property of persistent internal polarization after exposure to radiation and a polarizing voltage as a memory unit producing a photovoltaic output, comprising the steps of: producing mobile charges in said layer by irradiation from a source of substantial-1y uniform intensity; producing a charge separation of variable intensity by selectively applying an electric field to polarize only selected portions of the layer whereby only the polarized portions contain charge displacements approximately symmetrical with respect to the central plane of the layer between electrodes on opposite surfaces thereof and with charges of the same polarity collected at the surfaces on opposite sides of the layer under each electrode and to thereby store information in said memory unit; then reading out the stored information by irradiating the layer through an electrode with radiation of substantially uniform intensity and concomitantly detecting and measuring the current required to annihilate the charge separation as the output signal from the memory unit; and thereafter again selectively applying said electric field to produce the charge separation to thereby store information in said memory unit.

33. In combination with a layer of photoconductive insulating material exhibiting the property of persistent internal polarization after exposure to radiation and a polarizing voltage, there being electrodes on opposite sides of said layer, one of which is transparent to light emitted from a cathode ray tube; a cathode ray tube; means for directing the light emission from said cathode ray tube on to said layer through said transparent electrode; a source of polarizing voltage; a current sensing means; circuit means for connecting said voltage source to said electrodes for polarizing portions of said layer previously irradiated by light from said cathode ray tube and for connecting at least said transparent electrode to said current sensing means after said layer is polarized.

34. In combination with a layer of photoconductive insulating material exhibiting the property of persistent internal polarization after exposure to radiation and a polarizing voltage, there being electrodes on opposite sides of said layer, one of which is transparent to light emitted from a cathode ray tube; a cathode ray tube; means for directing the light emission from said cathode ray tube on to said layer through said transparent electrode; a source of polarizing voltage; a current sensing means; circuit means for connecting said voltage source to said electrodes for polarizing portions of said layer previously irradiated by light from said cathode ray tube and for connecting at least said transparent electrode to said current sensing means after said layer is polarized; horizontal and vertical sweep generator means for controlling the position of the light emission from said cathode ray tube; and signal detecting device connected to said current sensing means and to said sweep generator means for detecting the polarized portions of said layer.

References Cited by the Examiner UNITED STATES PATENTS NORMAN G. TORCHIN, Primary Examiner.

HAROLD N. BURSTEIN, Examiner. 

29. IN COMBINATION, A LAYER OF A PHOTOCONDUCTIVE INSULATING MATERIAL EXHIBITING THE PROPERTY OF PERSISTENT INTERNAL POLARIZATION AFTER EXPOSURE TO RADIATION AND A POLARIZING VOLTAGE; AN ELECTRODE STRUCTURE ON EACH SIDE OF SAID BODY COMPRISING A LAYER OF INSULATING MATERIAL AND A PLURALITY OF STRIPS OF ELECTRICALLY CONDUCTING MATERIAL, THE CONDUCTING MATERIAL BEING ON EACH SIDE OF SAID BODY BETWEEN SAID LAYER AND A SURFACE OF SAID BODY, THE STRIPS ON EACH ELECTRODE BEING PARALLEL TO AND SPACED FROM ONE ANOTHER AND LYING IN A DIRECTION PERPENDICULAR TO THE DIRECTION OF THE STRIPS ON THE OTHER ELECTRODE, THE ELECTRODE ON ONE SIDE OF SAID BODY BEING SUBSTANTIALLY TRANSPARENT TO RADIATION TO WHICH THE PHOTOCONDUCTIVE INSULATING MATERIAL IS PHOTO RESPONSIVE; MEANS FOR POPULATING ELECTRON TRAPS IN THE PHOTOCONDUCTIVE INSULATING BODY COMPRISING A SOURCE OF RADIATION POSITIONED TO IRRADIATE THE BODY THROUGH SAID RADIATION TRANSPARENT ELECTRODE; MEANS FOR PULLING ELECTRONS OUTWARDLY FROM 